Mobile communications networks are currently evolving from circuit switched (CS) networks towards packet switched (PS) networks, and by that integrate into Internet Protocol (IP) based infrastructures that are e.g. used for the Internet and the World Wide Web respectively.
So-called IP Multimedia Subsystem (IMS) networks have been developed for delivering multimedia services to mobile terminals (e.g. to GSM terminals being designed according to the well-known standard named Global System for Mobile Communications (GSM), or to Wideband Code Division Multiplex Access (WCDMA) stations. Hereto, calls from and to subscribers of the multimedia services using a circuit switched based access, in the following also being referred to as CS access, e.g. (e.g. WCDMA/GERAN) are routed through the IMS network in order to reach an IMS service engine. This concept is called IMS Centralized Services (ICS), e.g. being described the standardization document TS 23.292, (current release 9.1), established by the so-called Third Generation Partnership Project (3GPP) that is a standardization body to define globally applicable technical specifications.
A further 3GPP standard document in this scope is TS 23.237 (current release 9.0) specifying architectural requirements and procedures for delivery of PS-CS or CS-PS IMS Service Continuity.
In the frame of the 3GPP, further a project called Long Term Evolution (LTE) has been established to enhance the UMTS mobile phone standard to cope with future requirements. Within this project, the packet core is being developed to the so-called Evolved Packet Core (EPC) forming a part of the Evolved Packet System (EPS) that supports the so-called evolved UMTS Terrestrial Radio Access Network (eUTRAN) being a new packet oriented radio access (PS access network).
As part of the 3GPP standardization, e.g. being described in the document TS 23.216, (current release 8.3), work on call continuity for terminals equipped with single radio means (i.e. terminals with one single radio transmitter and one single radio receiver thus being capable of transmitting/receiving on only one of PS- or CS-access at a given time) being referred to as single radio voice call continuity (SR-VCC) is ongoing, enabling to transfer an IMS voice call from the EPS to the CS and vice versa.
According methods described in the above-cited document TS 23.216, single radio voice call continuity between PS access and CS access is based on the principle that an MSC Server being enhanced for SRVCC, also being referred to as evolved MSC server (eMSC), initiates a session transfer towards an application server being dedicated to maintain service continuity when handing over from an actual access network to a target access network, also being referred to as Service Centralization and Continuity Application Server (SCC-AS). Such handover might be initiated when the relocation request for the speech bearer is received from a mobility management entity (MME) that manages the mobility with respect to the mobile terminal. The target access network (e.g. the radio network controller RNC within the PS access network or the base station controller BSC within the CS access network) eventually executes the handover.
IMS Service Continuity is a solution that assumes an IMS service engine, so when the mobile terminal is attached via PS access, the respective call state is handled both in the mobile terminal and in the IMS. The CS access (i.e. CS radio and CS call control) serves merely as a vehicle to get to IMS (session control and services). An MSC server enhanced for ICS is acting as mobile terminal and may hence keep the call state as well.
According to methods described in the above-cited document TS 23.237 active/hold session are to be transferred individually between a so-called source access (transferring-out) leg and a target access leg (transferring-in) leg. Each session requires an own session transfer number or identifier. According to methods described in the above-cited document TS 23.216, the MSC server establishes a call towards the IMS based on a relocation request send by the mobility management entity (MME). The call is then handed over from PS to CS.
According to the existing 3GPP procedures, no information regarding any transaction is exchanged between the mobile terminal and the MSC; in other words, after handover the mobile terminal and the MSC server do not share sufficient transaction information. Thus a certain call cannot be referenced by any of both entities. As a consequence, only one active call can be transferred; any further call (e.g. held or waiting call) will be lost.